For each single vision ophthalmic lens intended to be worn in an eyeglass frame, there is an associated prescription for a given viewing distance. The ophthalmic prescription for this viewing distance can include a power value, positive or negative, as well as an astigmatism value and axis. This prescription compensates for the visual deficiency of the wearer, by means of the lens. The lens is mounted in the frame as a function of the position of the wearer's pupil relative to the frame, for the given viewing distance and viewing direction.
In the simplest case of conventional single vision lenses, the front and rear faces of these lenses are spherical surfaces, or toric if the prescription includes an astigmatism.
Advances have been made to improve the vision and increase the comfort of the wearer compared to conventional single view lenses.
A lens of the invention is single vision in the sense that it corresponds to a given single vision prescription, but this lens may present power and/or astigmatism variations around the prescribed values.
A first family of ophthalmic lens, of the improved single vision type and intended to correct the vision of a wearer, is known as the “aspheric lens” family. In this family, the optical quality of the glass is improved by optimizing the visual aberrations (for example deviations in power and astigmatism relative to the prescription) of the lens.
Another family of ophthalmic lens of the improved single vision type, intended to correct the vision of an ametropic non-presbyopic wearer, is disclosed in patent FR 2 871 247. The lenses described are adapted for esophoric ametropic non-presbyopic wearers, who experience visual fatigue when they are using their near vision for a prolonged period of time. A wearer may have heterophoria or phoria, meaning a deviation of the two visual axes relative to the point of fixation when binocular vision is dissociated. Phoria is evidenced as a modification of the visual axes when binocular vision is dissociated. In other words, in the active position—with binocular vision—the visual axes intersect at the point of fixation; in the passive position, in the absence of fusion stimulus or by eliminating any fusion and peripheral stimulation, the visual axes no longer pass through the point of fixation. For more details on the definition of phoria, one may consult Darras C., Eléments et réflexions d′optique physiologique (Elements and thoughts on physiological optics), ERA Edition, 1995.
The improved single vision ophthalmic lenses disclosed in patent FR 2 871 247 are specifically meant for wearers suffering from visual fatigue.